linux-kernel - Re: [RFC PATCH v4 3/6] sched/cpufreq: Hook em_pd_get_higher_power() into get_next

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Message-ID: <20200124143704.GA215244@google.com>
Date:   Fri, 24 Jan 2020 14:37:04 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Douglas Raillard <douglas.raillard@....com>
Cc:     linux-kernel@...r.kernel.org, rjw@...ysocki.net,
        viresh.kumar@...aro.org, peterz@...radead.org,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, linux-pm@...r.kernel.org
Subject: Re: [RFC PATCH v4 3/6] sched/cpufreq: Hook em_pd_get_higher_power()
 into get_next_freq()

On Thursday 23 Jan 2020 at 17:52:53 (+0000), Douglas Raillard wrote:
> We can't really move the call to em_pd_get_higher_freq() into
> cpufreq_driver_resolve_freq() since that's a schedutil-specific feature,
> and we would loose the !sg_policy->need_freq_update optimization.

Depends how you do it. You could add a new method to cpufreq_policy that
is defined only for sugov or something along those lines. And you'd call
that instead of cpufreq_frequency_table_target() when that makes sense.

> Maybe we can add a flag to cpufreq_driver_resolve_freq() that promises
> that the frequency is already a valid one. We have to be careful though,
> since a number of things can make that untrue:
>  - em_pd_get_higher_freq() will return the passed freq verbatim if it's
> higher than the max freq, so em_pd_get_higher_freq() will have to set
> the flag itself in case that logic changes.
>  - policy limits can change the value
>  - future things could tinker with the freq and forget to reset the flag.
> 
> If you think it's worth it I can make these changes.

The thing is, not only with the current patch we end up iterating the
frequencies twice for nothing, but also I think it'd be interesting to
use the EM for consistency with EAS. It'd be nice to use the same data
structure for the predictions we do in compute_energy() and for the
actual request.

Thoughts ?

Quentin